1. Field of the Invention
The present invention relates to an improvement in a continuous process for removing phosphatides and trace metals from crude oil by contacting the oil with acid.
2. Description of the Prior Art
In the processing of oils and fats for purposes of producing salad and cooking oils, and other edible oil products such as margarines and shortenings, and in the processing of triglyceride oils generally, the crude oil is usually alkali-refined. Often, a pretreatment of the crude oil with an acid, such as phosphoric acid, is applied to the oil before alkali-refining. The purpose of this acid-pretreatment is to achieve a more thorough removal of phosphatides or mucilaginous material from the crude oil than could be achieved by treatment with alkali alone. In cases where the crude oil is to be physically refined, that is, free fatty acids are to be removed from the oil in a steam-distillation or steam-refining operation rather than by alkali-refining, acid-pretreatment is particularly important. In such cases it is the only means for rendering phosphatides insoluble in the oil and hence subject to removal in subsequent bleaching operations conducted prior to physical refining. Bleaching is typically conducted by contact of the oil with an adsorbent substance such as an adsorbent clay.
If phosphatides are not thoroughly removed from the oil to a level of below about 10 ppm as P prior to deodorizing for edible use, the desired oil quality in respect to color, flavor, and flavor-stability cannot be achieved in the products. In processing for industrial uses, such as for alkyd-resins or soap-making, removal of phosphatides and other mucilaginous material is particularly important to achieve proper color stability in the products made from the crude oils.
In addition to the removal of phosphatides the acid-pretreatment of crude oils also serves the purpose of removing traces of heavy metals, notably iron and copper. Relatively high concentrations of iron often occur because of the inevitable contact of oils with iron in the course of extraction, storage and transport. Copper is not usually a problem because it can be avoided as a material of construction in extraction, storage and transport equipment. Oils which usually have relatively high free fatty acid concentrations (above 2%) such as crude palm oil, or palm-kernel oil and coconut oil are particularly likely to have significant concentrations of iron. Values in the range of 5-15 ppm are quite common. Iron levels in this range are quite detrimental to the color and flavor stability of oils. Processing must be capable of removing the iron. Experience by many investigators has shown that it is desirable to reduce the concentration to 0.2 ppm or less to avoid significant pro-oxidative effects resulting in poor color and poor flavor-stability as discussed above.
Because of the relatively high free fatty acid content of the above-mentioned oils, processing economics favor the use of steam-refining in place of alkali-refining. A pretreatment with phosphoric acid followed by a treatment with bleaching clay is usually applied to achieve the required removal of iron and other impurities including small amounts of phosphatides present in these oils.
Treating crude oils with phosphoric acid, or other acids, can of course be expected to have effects on an oil in addition to those just described. For instance, chlorophyll and related compounds are usually removed more easily from an oil after a phosphoric acid treatment, although, it should also be mentioned that other, weaker acids such as citric and oxalic are not effective in this respect. Further, it may be expected that products of oxidative breakdown reactions present in an oil can be affected by the acid-treatment. However, most of these effects are difficult to measure by analytical tests. It is, therefore, preferred when evaluating the total effect of the acid-treatment on an oil to process it to the end product, that is, through the alkali-refining, bleaching, and deodorizing operations, or, through bleaching and steam-refining/deodorizing, after the acid treatment. In the case of edible oils, evaluating the quality of the deodorized oil ensures that all effects, including those which cannot be determined analytically are measured.
In the pretreatment of crude oils with phosphoric acid as carried out in the industry, the acid is mixed with the oil or liquified fat (hereinafter designated generically as oil) at the desired contacting temperature and the mixture is held at that temperature for a sufficient amount of time to accomplish the desired reaction with phosphatides, heavy metals and other materials. The acid-pretreatment oil is then either immediately alkali-refined, or, if steam-refining/deodorizing of the oil is intended, is immediately treated with bleaching-clay. The removal of the acid-precipitate or acid-reacted material occurs together with the soapstock when the oil is subsequently alkali-refined, or with the spent bleaching-earth during filtration, when the acid-treated oil is treated with bleaching clay. It could, of course, also be removed centrifugally before alkali-refining or bleaching.
The acid contacting process is usually carried out in an agitated vessel, either batchwise, semi-continuously, or continuously. In the latter case, a compartmented and baffled vessel or reactor is usually used with an agitator in each compartment. The vessel is sized for an "average" residence-time similar to the "actual" residence-time required in a batch or semi-continuous operation. In most processing plants the continuous mode of operation is preferred because it is better suited for operation together with an alkali-refining or bleaching process, which are usually continuous operations.
The length of time required for the maximum effect of acid-oil contact is usually in the order of 15-30 minutes in batch, semi-continuous or continuous contacting apparatus. This contact-time, or residence-time, is required, at least in part, because of the insolubility of the acid in the oil. Another reason for the relatively long contact-times is that due to economic considerations, the amount of acid used must be as low as possible, yet accomplish removal of substantially all the phosphatides and metals. Moreover, in continuous operations, there is the added difficulty of short-circuiting and back-mixing of oil passing through the mixing vessel which often requires sizing of the vessel to allow an average residence-time greater than the residence-time required for a batch operation.
The limitations of conventional contacting equipment with respect to the degree of mixing which can be achieved and the relatively long contact-times required, present several disadvantages. Firstly, because of the long contact-times it is necessary to operate under vacuum or an inert gas, or to keep the reaction vessel flooded to ensure exclusion of air during the process to avoid oxidative damage to the oil. Secondly, changing the oil-stock to be processed is relatively cumbersome and results in a significant loss in processing capacity if done frequently. Thirdly, there are also general disadvantages stemming from building space, equipment costs, and acid usage. The object of the present process is to overcome these disadvantages.